Best Fish Tank Filters: A Practical Buying Guide

Choosing the best fish tank filter is one of the most important decisions you'll make for your aquarium, because filtration keeps water clean and safe for your fish between water changes. Every filter should perform three types of filtration: mechanical (trapping debris), biological (housing bacteria that break down ammonia and nitrite), and chemical (removing odours, discolouration or medications via media like activated carbon). The right choice depends largely on tank size and stocking level. Small to medium aquariums are often well served by an internal or hang-on-back filter, while larger tanks, heavily stocked tanks, or those with big or messy fish usually benefit from an external canister filter, which offers greater media capacity and stronger biological filtration. Alongside filtration, most tropical setups also need a reliable aquarium heater to maintain a stable temperature, since fluctuations stress fish and encourage disease. This guide walks through the main filter types, how to size one correctly, and answers to the questions we're asked most often, so you can choose equipment that matches your tank rather than over- or under-filtering it.

Internal vs External Fish Tank Filters

Internal filters sit inside the tank, are usually cheap and simple to run, and work well for nano and small-to-medium setups. Their downside is limited media capacity, which caps how much biological filtration they can support as your tank matures and stocking increases.

An external filter for a fish tank sits outside the tank, usually below or beside it, connected by inlet and outlet pipes. Because the canister can be much larger than anything that fits inside the aquarium, it holds significantly more mechanical, biological and chemical media — making external filters the go-to choice for medium-to-large aquariums, planted tanks, and setups with larger or waste-heavy fish. They're also quieter once primed and keep equipment out of view inside the tank.

Sizing Your Filter Correctly

Check the manufacturer's flow rate (litres per hour) against your tank's total water volume. A widely used starting point is turning over the full volume around 4-6 times per hour for a standard community tank, with higher turnover for fish that produce more waste or prefer stronger current. Undersized filtration is one of the most common causes of cloudy water and ammonia spikes, so when in doubt, size up rather than down.

Setting Up Filtration Alongside a Heater

Filtration and heating work together to create stable water conditions. Position your heater near the filter outlet so warmed water circulates evenly rather than settling in one area, and always allow a new filter to run through a full fishless cycling period before adding fish, so beneficial bacteria have time to establish on the media.

Maintaining Your Filter

Rinse mechanical and biological media in removed tank water rather than tap water to protect the bacterial colony, and stagger media replacement so you never remove all of it at once. If flow feels too strong for your fish or plants, an adjustable outlet or spray bar can help diffuse the current.

Frequently Asked Questions

What is the best type of fish tank filter?

There isn't one universal answer — internal filters suit small to medium tanks and are easy to install, while external (canister) filters are best for larger tanks or heavily stocked aquariums because they hold more media and provide stronger biological filtration. For most tanks over roughly 100-150 litres, an external filter for a fish tank tends to give the best long-term results.

How do I choose the right external filter for my aquarium?

Match the filter's rated flow (litres per hour) to your tank volume — as a general guide, aim for the filter to turn over the total water volume around 4-6 times per hour, and go higher if you keep fast-swimming or heavily stocked species. Also check it has enough media baskets for mechanical, biological and chemical stages.

Do I need both a filter and a heater for a fish tank?

Yes, in almost all tropical freshwater setups. The filter keeps water chemically and biologically safe, while the heater keeps temperature stable for species that need warmer, consistent conditions; coldwater-only aquariums may not need a heater, but check the needs of your specific fish.

How often should I clean or replace fish tank filter media?

Rinse mechanical and biological media in removed tank water (never tap water) every 2-4 weeks to avoid killing beneficial bacteria, and replace chemical media like carbon roughly monthly since it stops absorbing over time. Avoid replacing all media at once, as this can cause a mini-cycle and ammonia spike.

Can an external filter be too powerful for my tank?

Yes — excessive flow can stress fish, especially slower swimmers, and disturb substrate or plants. Most external filters have an adjustable flow valve, or you can add a spray bar to diffuse output across the tank.

What size heater does my fish tank need?

A common rule of thumb is around 1 watt per litre of tank water for a typical room temperature, though larger tanks or colder rooms may need proportionally more. Always check the heater is rated for your exact tank volume and use a thermometer to confirm stable temperature.